Switched capacitor DRAM sense amplifier with immunity to mismatch and offsets

ABSTRACT

A switched capacitor sense amplifier includes capacitively coupled input, feedback, and reset paths to provide immunity to the mismatches in transistor characteristics and offsets. The sense amplifier includes a cross-coupled pair of inverters with capacitors absorbing offset voltages developed as effects of the mismatches. When used in a dynamic random access memory (DRAM) device, this immunity to the mismatches and offsets allows the sense amplifier to reliably detect and refresh small signals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/931,552, filed Aug. 31, 2004, now U.S. Pat. No. 7,221,605,which is incorporated herein by reference.

TECHNICAL FIELD

This document generally relates to memory sense amplifiers andparticularly, but not by way of limitation, to a cross-coupled senseamplifier with a switched capacitor circuit configured to compensate foreffects of transistor mismatches and offsets.

BACKGROUND

Sense amplifiers are used in memory devices to allow for reduced voltageswing on the bit lines. In a dynamic random access memory (DRAM)circuit, each data bit is stored in a small storage capacitor that isdischarged quickly. A sense amplifier detects a signal representing thebit on a bit line and amplifies the signal to an amplitude near the DRAMcircuit's supply voltage. The capacitor is recharged as the signal isamplified. The data bit is refreshed before it ceases to be detectableas the sense amplifier detects and amplifies the signal on a periodicbasis, such as every few milliseconds.

Cross-coupled sense amplifiers are among various sense amplifierconfigurations used in DRAM circuits. A known cross-coupled senseamplifier includes a pair of inverters “cross coupled” between acomplementary pair of bit lines. Each inverter has its input connectedto one bit line and its output connected to the complementary bit line.A reset switch, when closed, causes both bit lines to be precharged toabout one half of the DRAM circuit's supply voltage by connecting thebit lines to each other and to all the inputs and outputs of bothinverters. A cross-coupled sense amplifier provides for fast signalamplification for the DRAM circuits. However, in practice it isdifficult to provide the pair of inverters with perfectly matchedtransistors. Mismatches in transistor characteristics may produce, forexample, an offset voltage across the outputs of the inverters duringthe reset. This offset is reflected to the inputs of the inverters. Inthe worst case, this reflected offset is detected as a signalrepresenting a data bit after the reset, resulting in a data error.

Therefore, there is a need to provide immunity to the mismatches intransistor characteristics and offsets while maintaining the fastresponse of cross-coupled sense amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals describe similar components throughoutthe several views. The drawings illustrate generally, by way of example,but not by way of limitation, various embodiments discussed in thepresent document.

FIG. 1 is a schematic/block diagram illustrating an embodiment ofportions of a memory circuit including a switched capacitor senseamplifier.

FIG. 2 is a schematic illustrating an embodiment of the switchedcapacitor sense amplifier.

FIG. 3 is a schematic illustrating a specific embodiment of the switchedcapacitor sense amplifier.

FIG. 4 is a graph showing simulation results illustrating theperformance of the switched capacitor sense amplifier of FIG. 3.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description provides examples,and the scope of the present invention is defined by the appended claimsand their equivalents.

It should be noted that references to “an”, “one”, or “various”embodiments in this document are not necessarily to the same embodiment,and such references contemplate more than one embodiment.

In this document, “AC” refers to “alternating current.” “DC” refers to“direct current.” An “MOS transistor” or “MOSFET” refers to ametal-oxide semiconductor field-effect transistor. An “NMOS transistor”refers to an n-channel metal-oxide semiconductor field-effect transistor(or n-channel MOSFET). A “PMOS” refers to a p-channel metal-oxidesemiconductor field-effect transistor (or p-channel MOSFET). Each MOStransistor (either NMOS or PMOS transistor) has a gate terminal, a drainterminal, and a source terminal.

This document discusses, among other things, a switched capacitor senseamplifier that improves the accuracy of a cross-coupled sense amplifierby providing immunity to mismatches in transistor characteristics andoffsets. Instead of a DC-coupled reset path connecting all inputs andoutputs of the cross-coupled pair of inverters to the complementary pairof bit lines when the reset switch is closed, as in the knowncross-coupled sense amplifier, an AC-coupled reset path is coupledbetween the complementary bit lines. Capacitors in the AC-coupled resetpath absorb offset voltages developed in the sense amplifier circuit dueto the mismatches in transistor characteristics. Thus, the offsetvoltages are substantially prevented from appearing in the bit lines andbeing detected as signals. When compared to the known cross-coupledsense amplifier, the elimination of the effects of the mismatches intransistor characteristics and offsets provides the switched capacitorsense amplifier with a better sensitivity, i.e., the ability to reliablydetect smaller signals.

FIG. 1 is a schematic/block diagram illustrating an embodiment ofportions of a memory circuit including switched capacitor senseamplifiers 100. In one embodiment, the memory circuit is a DRAM circuit.The memory circuit includes a memory array 101 including rows andcolumns of memory cells 102. As illustrated in FIG. 1, memory array 101has m rows and n columns, with pairs of complementary bit linesBL0/BL0*-BLm/BLm* and word (address) lines WL0-WLn. Each of memory cells102 is identified by one unique combination of a bit line BL (selectedfrom BL0-BLm) or BL* (selected from BL0*-BLm*) and a word line WL(selected from WL0-WLn).

Complementary bit line pairs BL0/BL0*-BLm/BLm* are used for writing datainto and reading data from memory cells 102. Word lines-WL0-WLn areaddress lines used for selecting the memory cells to which data arewritten into and from which the data are read from. Address buffers 106receive address signals A0-An from address lines 105 connected to anexternal controller, such as a microprocessor coupled to the memorycircuit. In response, address buffers 106 control row decoders 107 andcolumn decoder and input/output circuitry 108 to access memory cells 102selected according to address signals A0-An. Data provided at datainput/outputs 109 are capable of being written into memory array 101.Data read from memory array 101 are applied to data input/outputs 109.Memory cells 102 each include a switch 103 and a storage capacitor 104.In one embodiment, switch 103 includes an n-channel field effecttransistor, such as an NMOS transistor. The n-channel transistor has adrain terminal coupled to a BL (selected from BL0-BLm) or a BL*(selected from BL0*-BLm*), a source terminal coupled to storagecapacitor 104, and a gate terminal coupled to a WL (selected fromWL0-WLn).

To write or read data, address buffers 106 receive an addressidentifying a column of memory cells and select one of the word linesWL0-WLn according to the address. Row decoder 107 activates the selectedword line to activate switch 103 of each cell connected to the selectedword line. Column decoder and input/output circuitry 108 selects theparticular memory cell for each data bit according to the address. Towrite data, each date bit at data input/outputs 109 causes storagecapacitor 104 of one of the selected cells to be charged, or to staydischarged, to represent the data bit. To read data, a data bit storedin each of the selected cells, as represented by the charge state ofstorage capacitor 104 of the selected cell, is transferred to datainput/outputs 109.

Switched capacitor sense amplifiers 100 are each coupled between acomplementary bit line pair, BL and BL*. Storage capacitor 104 in eachof memory cells 102 has a small capacitance and holds a data bit for alimited time as the capacitor discharges. Switched capacitor senseamplifiers 100 are used to “refresh” memory cells 102 by detecting andamplifying signals each representing a stored data bit. The amplifiedsignals recharge the storage capacitors and hence maintain the data inmemory cells 102. As discussed in detail below, switched capacitor senseamplifiers 100 each include a cross-coupled pair of inverters with aswitched capacitor circuit providing for immunity to transistorcharacteristic mismatches and offsets.

FIG. 2 is a schematic illustrating an embodiment of a switched capacitorsense amplifier 200. Sense amplifier 200 is coupled between acomplementary pair of bit lines BL and BL*. In one embodiment, senseamplifier 200 is used as one of switched capacitor sense amplifiers 100illustrated in FIG. 1. The pair of bit lines BL and BL* represents anypair of complementary bit lines BL0/BL0*-BLm/BLm* as illustrated inFIG. 1. However, the application of sense amplifier 200 is not limitedto the circuit illustrated in FIG. 1.

Sense amplifier 200 includes a bit line node 210 coupled to BL and a bitline node 212 coupled to BL*. A capacitor 214, coupled between BL and aground node 290, represents the bit line capacitance associated with BL.Another capacitor 216, coupled between BL* and ground node 290,represents the bit line capacitance associated BL*. The bit linecapacitance, i.e., the capacitance of each of capacitors 214 and 216, istypically around 1 picofarad.

Sense amplifier 200 detects signals representing bits on bit lines BLand BL* and amplifies the voltage across bit lines BL and BL* (i.e., thedifference between the amplitude of the signal at bit line node 210 andthe amplitude of the signal at bit line node 212). A cross-coupled pairof inverters 220 and 222 performs the detection and amplification.Inverter 220 has its input capacitively coupled to bit line node 210 andits output directly coupled to bit line node 212. Inverter 222 has itsinput capacitively coupled to bit line node 212 and its output directlycoupled to bit line node 210. An input capacitor 224 is coupled betweenbit line node 210 and an input node 240, which is coupled to the inputof inverter 220. Input capacitor 224 provides the capacitive couplingbetween bit line node 210 and the input of inverter 220. Another inputcapacitor 226 is coupled between bit line node 212 and an input node242, which is coupled to the input of inverter 222. Input capacitor 226provides the capacitive coupling between bit line node 212 and the inputof inverter 222. In one embodiment, input capacitors 224 and 226 eachhave a capacitance in a range of approximately 0.1 to 1.0 picofarads. Aswitched capacitor feedback-reset circuit, which includes a reset switchand a capacitor connected in parallel, is coupled between the input andthe output of each of inverters 220 and 222. A feedback capacitor 228 iscoupled between bit line node 212 and input node 240, i.e., the outputand the input of inverter 220, to provide for a capacitively coupledfeedback path for inverter 220. A reset switch 232 is coupled betweenbit line node 212 and input node 240, i.e., the output and the input ofinverter 220, to allow for reset of inverter 220 by equalizing thepotentials at its input and output. Another feedback capacitor 230 iscoupled between bit line node 210 and input node 242, i.e., the outputand the input of inverter 222, to provide for a capacitively coupledfeedback path for inverter 222. Another reset switch 234 is coupledbetween bit line node 210 and input node 242, i.e., the output and theinput of inverter 222, to allow for reset of inverter 222 by equalizingthe potentials at its input and output. In one embodiment, feedbackcapacitors 228 and 230 each have a capacitance in a range ofapproximately 0.01 to 0.1 picofarads. The switched capacitorfeedback-reset circuits allow inverters 220 and 222 to be separately andindependently reset.

When switches 232 and 234 are closed to reset inverters 220 and 222 andprecharge bit lines BL and BL* and the inputs of inverters 220 and 222,voltages developed across input capacitors 224 and 226 compensate forthe effects of mismatches in transistor characteristics and offsets thatwould otherwise be seen on bit lines BL and BL*. In other words, theoffset voltages developed as the effects of the mismatches in transistorcharacteristics are absorbed by input capacitors 224 and 226 instead ofbeing applied onto bit lines BL and BL*. When switches 232 and 234 areopened after the reset, a signal is applied to bit lines nodes 210 and212 and is capacitively coupled to the input of each of inverters 220and 222. This causes the input voltage of one of inverters 220 and 222to be driven above its equilibrium value and the input voltage of theother inverter to be driven below its equilibrium value. Mismatches inthe equilibrium voltage between inverters 220 and 222 do notsubstantially affect the output and the operation of sense amplifier200.

When compared to the known cross-coupled sense amplifier that includesinverters with their inputs and outputs directly coupled to the bitlines, sense amplifier 200 has a better sensitivity for detectingsignals on bit lines BL and BL*. If the effects of transistor mismatchesand offsets are seen as a voltage across bit lines BL and BL*, they maybe detected and amplified as a signal representing a bit and thus causea data error. By using input capacitors 224 and 226 to absorb theeffects of transistor mismatches and offsets, sense amplifier 200 isable to detect signals with smaller amplitudes, knowing they are noteffects of the mismatches and offsets.

FIG. 3 is a schematic illustrating a switched capacitor sense amplifier300 as a specific embodiment of sense amplifier 200. Sense amplifier 300has the basic configuration of sense amplifier 200, with the invertersand reset switches implemented with CMOS technology.

An inverter 320, which is a specific embodiment of inverter 220,includes a complementary pair of a PMOS transistor 350 and an NMOStransistor 352. The gate terminals of transistors 350 and 352 arecoupled together as the input of inverter 320, which is then coupled toinput node 240. The drain terminals of transistors 350 and 352 arecoupled together as the output of inverter 320, which is then coupled tobit line node 212. The source terminal of transistor 350 is coupled to apower supply node 392 (VDD). The source terminal of transistor 352 iscoupled to ground node 290.

Another inverter 322, which is a specific embodiment of inverter 222,includes a complementary pair of a PMOS transistor 354 and an NMOStransistor 356. The gate terminals of transistors 354 and 356 arecoupled together as the input of inverter 322, which is then coupled toinput node 242. The drain terminals of transistors 354 and 356 arecoupled together as the output of inverter 322, which is then coupled tobit line node 210. The source terminal of transistor 354 is coupled topower supply node 392 (VDD). The source terminal of transistor 356 iscoupled to ground node 290.

A reset switch 332, which is a specific embodiment of reset switch 232,includes an NMOS transistor 358. Transistor 358 has its drain terminalcoupled to the output of inverter 320 (bit line node 212) and its sourceterminal coupled to the input of inverter 320 (input node 240). A resetcontrol line 304 is coupled to the gate terminal of transistor 358.

Another reset switch 334, which is a specific embodiment of reset switch234, includes an NMOS transistor 360. Transistor 360 has its drainterminal coupled to the output of inverter 322 (bit line node 210) andits source terminal coupled to the input of inverter 322 (input node242). Reset control line 304 is coupled to the gate terminal oftransistor 360.

FIG. 4 is a graph showing simulation results illustrating theperformance of switched capacitor sense amplifier 300. As one specificembodiment, as well as for the purposes of analysis and simulation, eachcircuit element of sense amplifier 300 is assigned one or morecharacteristic parameters. These parameters are given by way of example,but not by way of limitation.

A reset pulse is applied to reset control line 304 to reset inverters320 and 322 simultaneously (while separately and independently). Voltagesources are introduced between capacitor 214 and ground node 290, andbetween capacitor 216 and ground node 290, to introduce signals to bitlines nodes 210 and 212. A signal 401, representing a data bit, isapplied to bit line node 210, and a complementary signal 402 is appliedto bit line node 212. As seen in FIG. 4, the signal amplitude is 100millivolts across bit lines nodes 210 and 212. Capacitors 214 and 216each have a capacitance (i.e., the bit line capacitance) of 1 picofarad.For the purpose of simulation, 1-ohm resistors each representing a bitline resistance are introduced between bit line node 210 and the outputof inverter 322 and between bit line node 212 and the output of inverter320. Also for the purpose of simulation, 10-megaohm resistors arecoupled between the input and output of each of inverters 320 and 322,between the input of each of inverters 320 and 322 and ground node 290,and between each of bit lines 210 and 212 and ground node 290. Inputcapacitors 224 and 226 each have a capacitance of 1 picofarad. Feedbackcapacitors 228 and 230 each have a capacitance of 0.01 picofarads. Tointroduce transistor mismatches, transistors 352 and 356 are givensubstantially different threshold voltages. For the purpose ofsimulation and analysis with a specific example, transistor 352 has athreshold voltage that is substantially higher than that of transistor356. Other characteristic parameters of transistors 352 and 356 arematched. Transistors 350 and 354 are matched transistors. A 1.5-voltvoltage is applied to power supply node 392 (VDD).

Transistor 356 has a substantially lower threshold voltage and istherefore substantially more conductive than transistor 352. After thereset, the voltage at the output of inverter 322 (bit line node 210) islower than the voltage at the output of inverter 320 (bit line node212). Bit line node 210 is precharged to a voltage lower than VDD/2 andlower than the voltage at the input of inverter 320 (node 240, whichequals the voltage at the output of inverter 320 and is higher thanVDD/2). The voltage difference between bit line node 210 and input node240 (the potential-equalized input and output of inverter 320) isapplied across input capacitor 224. The voltage at the output ofinverter 320 (bit line node 212) is higher than the voltage at theoutput of inverter 322 (bit line node 210). Bit line node 212 isprecharged to a voltage higher than VDD/2 and higher than the voltage atthe input of inverter 322 (node 242, which equals the voltage at theoutput of inverter 322 and is lower than VDD/2). The voltage differencebetween bit line node 212 and input node 242 (the potential-equalizedinput and output of inverter 322) is applied across input capacitor 226.

Curves 403 and 404 show simulation results with a 400-millivolt mismatchin threshold voltage between transistors 352 and 356. Curve 403 is thevoltage at the output of inverter 320 (bit line node 212), and curve 404is the voltage at the output of inverter 322 (bit line node 210). Curves403 and 404 show that sense amplifier 300 correctly detects andamplifies a 100-millivolt signal even though transistors 352 and 356 aremismatched by a 400-millivolt threshold voltage. Curves 403 and 404 alsoshow that the output voltage swing across bit lines 210 and 212 reaches50% of the rail-to-rail voltage at about 3 nanoseconds.

In general, this document discusses, among other things, a switchedcapacitor sense amplifier that includes capacitively coupled input,feedback, and reset paths to provide immunity to the mismatches intransistor characteristics and offsets. The sense amplifier includes across-coupled pair of inverters with capacitors absorbing offsetvoltages developed as effects of the mismatches. When used in a DRAMdevice, this immunity to the mismatches and offsets allows the senseamplifier to reliably detect and refresh small signals.

In one embodiment, a memory circuit includes a complementary pair offirst and second bit lines. A switched capacitor sense amplifier iscoupled between the first and second bit lines. The switched capacitorsense amplifier includes a cross-coupled pair of first and secondinverters. The input of the first inverter is AC-coupled to the firstbit line through a first input capacitor. The output of the firstinverter is coupled to the second bit line. The input of the secondinverter is AC-coupled to the second bit line through a second inputcapacitor. The output of the second inverter is coupled to the first bitline. A first feedback capacitor and a first reset switch are eachcoupled between the input and output of the first inverter. A secondfeedback capacitor and a second reset switch each are coupled betweenthe input and the output of the second inverter. When the first andsecond reset switches are closed to reset the switched capacitor senseamplifier, the input and output of each of the first and secondamplifiers are equalized. Voltages developed across the first and secondinput capacitors compensate for offset voltages developed in the senseamplifier circuit due to the mismatches in transistor characteristics.The offset voltages are therefore substantially prevented from appearingin the bit lines and being detected as signals.

In one embodiment, a method for resetting a sense amplifier inpreparation for sensing a memory state of a DRAM cell is provided. Thesense amplifier includes a first inverter, a first input capacitorcoupling a first bit line and an input of the first inverter, a secondinverter, and a second input capacitor coupling a second bit line and aninput of the second inverter. The second bit line is coupled to theinput of the first inverter to place a potential of the second bit lineat the input of the first inverter such that a voltage differencebetween the first and second bit lines is placed across the first inputcapacitor. The first bit line is coupled to the input of the secondinverter to place a potential of the first bit line at the input of thesecond inverter such that a voltage difference between the second andfirst bit lines is placed across the second input capacitor.

This disclosure includes several processes, circuit diagrams, andstructures. The present invention is not limited to a particular processorder or logical arrangement. Although specific embodiments have beenillustrated and described herein, it will be appreciated by those ofordinary skill in the art that any arrangement which is calculated toachieve the same purpose may be substituted for the specific embodimentsshown. This application is intended to cover adaptations or variations.It is to be understood that the above description is intended to beillustrative, and not restrictive. Combinations of the aboveembodiments, and other embodiments, will be apparent to those of skillin the art upon reviewing the above description. The scope of thepresent invention should be determined with reference to the appendedclaims, along with the full scope of equivalents to which such claimsare entitled.

1. A method for resetting a sense amplifier in preparation for sensing amemory state of a dynamic random access memory (DRAM) cell, the senseamplifier including a first inverter, a first input capacitor coupling afirst bit line and an input of the first inverter, a second inverter,and a second input capacitor coupling a second bit line and an input ofthe second inverter, the method comprising: coupling the second bit lineto the input of the first inverter to place a potential of the secondbit line at the input of the first inverter such that a voltagedifference between the first and second bit lines is placed across thefirst input capacitor; and coupling the first bit line to the input ofthe second inverter to place a potential of the first bit line at theinput of the second inverter such that a voltage difference between thesecond and first bit lines is placed across the second input capacitor.2. The method of claim 1, wherein coupling the second bit line to theinput of the first inverter comprising closing a first reset switchcoupled between the second bit line and the input of the first inverter,and coupling the first bit line to the input of the second invertercomprising closing a second reset switch coupled between the first bitline and the input of the second inverter.
 3. The method of claim 2,further comprising: coupling the output of the first inverter to theinput of the first inverter through a first feedback capacitor when thefirst reset switch is open; and coupling the output of the secondinverter to the input of the second inverter through a second feedbackcapacitor when the second reset switch is open.
 4. The method of claim2, wherein closing the first reset switch and closing the second resetswitch comprise applying a reset pulse to a first gate terminal of afirst MOS transistor of the first reset switch and a second gateterminal of a second MOS transistor of the second reset switchsimultaneously.
 5. The method of claim 1, further comprising absorbing avoltage offset between the first bit line and the second bit line usingthe first and second input capacitors, the voltage offset associatedwith at least one mismatch of transistor characteristics between thefirst and second inverters.
 6. The method of claim 1, comprisingproviding the first and second input capacitors each being a capacitorhaving a capacitance between approximately 0.1 and 1.0 picofarads. 7.The method of claim 1, comprising providing the first and secondinverters each being a CMOS inverter.
 8. The method of claim 2, whereincoupling the second bit line to the input of the first invertercomprises coupling a drain terminal of a first NMOS transistor to thesecond bit line and a source terminal of the first NMOS transistor tothe input of the first inverter, and coupling the first bit line to theinput of the second inverter comprises coupling a drain terminal of asecond NMOS transistor to the first bit line and a source terminal ofthe second NMOS transistor to the input of the second inverter.
 9. Themethod of claim 8, comprising coupling gate terminals of the first andsecond NMOS transistors to a reset control line.
 10. The method of claim8, comprising coupling a first feedback capacitor between the second bitline to the input of the first inverter, and coupling a second feedbackcapacitor between the first bit line to the input of the secondinverter.
 11. The method of claim 10, comprising providing the first andsecond feedback capacitors each having a capacitance between 0.01 and0.1 picofarads.